Implement redesign, refactor everything

- Write CSS for everything, update templates.
- Refactor discussion view. Stream is split into two components
(content and scrubber) which have their own responsibilities.
- Extract pane functionality into a mixin.
- Implement global “back button” system. You give a “paneable” target
to the application controller, the back button will modulate its
pane-related properties as necessary, and call an action when the
button is clicked.
- Extract welcome-hero into its own component.
- Lots of other general improvements/refactoring. The code is quite
well-commented so take a look!

ember/app/controllers/composer.js

@@ -2,7 +2,9 @@ import Ember from 'ember';
 
 export default Ember.Controller.extend({
 
-    needs: ['discussions'],
+    needs: ['index'],
+
+    user: Ember.Object.create({avatarNumber: 1}),
 
     showing: false,